This invention relates to tire-changing machines and more particularly to such machines in which the wheel is mounted on a table and the tire bead is broken by a pair of oppositely acting bead-breaking shoes.
There have always been problems with the lower bead-breaking shoe.
In normal operation with certain machines of the prior art the wheel rim with tire mounted is seated on a table. The table top is either sloped or curved to accomodate various diameters of wheels. The wheel sits on this top with a lower rim projecting laterally therefrom. When the tire to be changed is mounted on the table top the lower bead-breaking shoe is obscured from the operators view by the tire itself.
The result has been that these shoes of the prior art, in their bead-breaking motion, if the attitude of the shoe is not set properly, will engage the wheel rim or a structure known as a safety hump on the wheel well wall.
This engagement may stall the operation of the machine or result in bending or breaking of the wheel rim or the wheel.
Present day tires and wheels are provided with many contours and sizes.
The advent of the radial tire with its different structure and profile makes such tires difficult to remove from wheel rims.
The wheels, particularly alloy wheels, are softer and more susceptible to damage because of their width and composition.
The wider rims are more difficult not to engage. In certain mechanisms of the prior art it was proposed that the movement of the lower shoe be at a constant distance from the table top; in some structures such as the Foster U.S. Pat. No. 3,158,190 two shoes were provided. These mechanisms assumed that the operator set the lower bead-breaking shoe at the correct attitude so that there was no engagement with the rim.
However, frequently, if the shoe does not engage the rim it does not engage the bead at the correct angle.
It should be understood that the most efficient engagement of the shoe with tire to effect bead-breaking is at the junction of the bead and the wheel rim. If the shoe engages the tire on the side wall the bead-breaking energy is dissipated or absorbed by the flexible characteristics of the side wall. And if the shoe engages the side wall first then it will through the friction of the side wall be engaged and remain engaged in the attitude at which it initially engaged.
The absorption of the bead-breaking shoe force by the rubber of the tire is a problem with all forms of bead-breaking shoe and in the past the configuration of the shoe itself has not assisted.
As a consequence of the dissipation of the bead-breaking force by the rubber absorbing the energy there has been an increased need to provide power to overcome the energy loss.
In U.S. Pat. No. 3,158,190 the problem of two-ply tires is discussed. With radial tires the difficulties are increased and with the increased cost of such structures damage to the side wall has to be minimized while at the same time more power has to be provided which is more effectively employed.
As mentioned previously the shoe itself has been of minimum assistance in what may be termed the delivery of the bead-breaking force.
Generally, the shoes of the prior art are of such a form that there is substantially simultaneous engagement of the leading edge of the shoe about an arc of the bead. As a result the bead-breaking force is delivered about an extended area of the perimeter of the bead and the force is substantially vertical about that area. This type of force application again results in the stalling of machines. This stalling in certain machines can result in a "whip-lash" effect with the result that an operator frequently finds himself subject to assault by the tire iron used to remove the tire.